Terahertz radiation still lies in a metrological no man's land--a "metrology gap." The Physikalisch-Technische Bundesanstalt (PTB; Braunschweig, Germany) can now close this gap. For the first time, a commercial terahertz laser was traced back to the international system of units (SI) by measuring its output power absolutely. PTB achieved this with a power meter that had been calibrated beforehand against a cryogenic radiometer that is Germany's primary standard for power measurement of electromagnetic radiation. As a result, the terahertz laser--a model SIFIR-50 manufactured by Coherent (Santa Clara, CA)--became the first terahertz laser in the field with an output power calibrated to the SI system.
Knowledge of power is power
Terahertz radiation is used for spectroscopy, analytical science and astronomy. As technological innovation has improved the generation and detection of terahertz radiation, more and more areas have been added: testing of materials, security checks at airports, biological and medical science, quality inspection of foodstuffs and agricultural goods, global environmental monitoring, and information and communication techniques. All of these applications benefit from reliable power measurement in the terahertz region.
The Coherent SIFIR-50 laser was operated at 2.5 THz for the first power measurement. The laser is the core instrument of a new facility for the determination of terahertz radiant power in SI units and for the calibration of terahertz detectors. It is a far-IR molecular gas laser pumped by an integrated, frequency stabilized, tunable 50 W carbon dioxide (CO2) laser.
"The unique feature of the system is its Fabry-Perot lock used to control the operating frequency of the CO2 laser, which is optimal for pumping the vibrational transitions of the gas molecules," says Andreas Steiger of PTB Berlin. Combined with its thermally-compensated terahertz resonator and pump-optics design, the laser delivers stable terahertz radiation, tunable from 1 to 7 THz, to several rotational transitions of the molecules in use. The tuning range contains a variety of discrete terahertz lines for many different gases. "The performance and beam quality of the laser enables PTB to take a large step in bridging the metrology gap between ultrahigh-frequency electronics and far-IR optics in the near future," adds Steiger.
